The invention generally relates to carbide bits used in rotary tools by the electronics industry in printed circuit board (hereinafter "PCB") fabrication and, more particularly, to a cryogenic tempering process for extending the useful life of such PCB tool bits.
The representative tool bit of this class is a true drill bit, as used exclusively for axial boring. PCB drill bits range in diameter between about 20/10,000-ths of an inch (0.0020 inches) and 1/4-th of an inch (0.250 inches). However, two other members of this class of tool bits for rotary tools of PCB fabrication include: end mills and router bits. None of these three kinds of rotary-tool bits--ie., true drill bits, end mills or router bits--is generally ever any larger than 1/4-th of an inch (0.250 inches) in diameter in the PCB fabrication field. Also, they are fairly similar in configuration. For convenience in this description, the phraseology "drill bit" predominantly is used to designate the general class of these tool bits for rotary tools.
Unless the context makes it clear otherwise, there will be only few occasions where the "drill bit" tool bit under discussion is only specifically a true drill bit:--eg., a tool bit used for axial boring only. Again, generally, the phrase "drill bit" as used herein is predominantly non-limiting in that it applies equally as well among true drill bits, end mills and router bits, as used in the electronics industry for PCB fabrication. Thus "drill bit" and "tool bit" are often used interchangeably.
The cryogenic tempering process in accordance with the invention is performed with equipment and machinery which is conventional in the thermal cycling treatment field. First, the articles-under-treatment are placed in a treatment chamber which is connected to a supply of cryogenic fluid, such as liquid nitrogen or a similar low temperature fluid. Exposure of the chamber to the influence of the cryogenic fluid lowers the temperature until the desired level is reached. In the case of liquid nitrogen, this is about -300.degree. F. (ie., 300.degree. F. below zero).
PCB's typically but not exclusively are panels of "fiberglass" which more particularly is a composition of glass and phenolic. Fiberglass as well as other typical compositions used in PCB manufacture simply place high demands on drill bits. PCB material, fiberglass or otherwise, is generally always very abrasive. It dulls drill bits relatively quickly. A drill bit that is dulled until it fails to meet tolerance standards must be immediately replaced. Briefly, as background, the machining operations on PCB'S must be precise and match very close tolerances. For true drill bits or end mills, to give an example, the tolerances are measured in respect of bore diameter, axial straightness, and depth of bore. The PCB's are typically stacked for drilling operations. That way many boards or layers are drilled at once. The stop means provided to stop the depth of the bore is usually formed directly on a true drill bit; it may be a collar that provides a stop shoulder. Such stop collars are located on the drill bits with likewise very exacting tolerances. Typically the span between tip and the shoulder is measured and originally set by a laser device. It is that precise.
Hence, this drilling/fabricating environment not only requires very close precision or tight tolerances, but it is also carried out on a material which is highly abrasive. Accordingly, the majority of tool bits used in this environment are hardened carbide steel so as not to dull as quickly. With conventional carbide PCB drill bits, users are getting between about 500 and 2,000 cycles out of each drill bit before it is so dull it is spent. Spent true drill bits are typically replaced with fresh ones and discarded after being sharpened three times. Re-sharpening router bits and end mills has never proven practical because of cost of sharpening while maintaining tolerances.
What is needed is an improvement which will extend the use life of such PCB tool bits beyond the prior art benchmark of, say, 500 to 2,000 cycles or so.
Certain formats of cryogenic treatment are known for extending the wearability of various steel alloy articles. For instance, the U.S. patent to Nu-Bit, Inc., U.S. Pat. No. 5,259,200--Kamody discloses particular format of a cryogenic treatment for drill bits:--large drill bits.
According to Kamody, the state of the prior art at the time of his invention practiced by the following convention:
As is apparent from the above description, the time period necessary to complete each step in the cycle of the treatment process generally is a minimum of about an hour per cross-section inch of the article being treated. Thus, for example, treatment of a steel article having a one inch cross-section in the minimum dimension would require a minimum of four hours total to complete the treatment according to generally accepted practices. In a like fashion, an article having a three inch minimum cross-section dimension would require a minimum of twelve hours total to complete the treatment according to the same accepted practices. However, it has been fairly conventional to increase the time periods for each step of the process to ensure that treatment is complete. Thus, for example, many of those practicing the above process routinely provide a safety factor of two or three or more in determining the respective time periods for the steps and as a consequence, overall treatment time periods of up to 50 hours or more for an article having a cross-sectional minimum dimension of one inch are often used. In using such extended time periods for the cryogenic treatment, it is believed that possible stress cracking and distortion of the article are thereby minimized or even eliminated. U.S. Pat. No. 5,259,200. PA1 Generally, the commercial economics of metallurgical procedures dictate that a particular treatment should be accomplished as quickly as possible so as to minimize the size of the equipment necessary and thus equipment costs as well as requiring less space, energy and inventory in processing.*** Thus, for example, a tool steel article having a minimum cross-sectional dimension of about four inches, the maximum time for treatment [in accordance with Kamody's discovery] of the article in the bath of cryogenic fluid would be about ten minutes. U.S. Pat. No. 5,259,200.
However, Kamody's personal inventive efforts are directed at reducing such process time.
Another format of a cryogenic process for extending the wearability of a steel article is disclosed by U.S. Pat. No. 5,865,913--Paulin et, al., for firearm barrels. This patent for treatment of firearm barrels can be taken as representative of various others still.
In general, cryogenic process is popular for steel alloys because it improves the resistance of metal to normal wear and tear. It is speculated that cryogenic processes affect the wearability of steel by four known mechanisms:--conversion of austenite to martensite; precipitation hardening which may increase Rockwell hardness; formation of fine carbide particles; and residual stress relief. Whether the mechanics are truly known, actual trials on numerous articles bears witness to cryogenics efficacy. Thus, in the case of firearm barrels, "the accuracy of a firearm is directly tied to the heat generated by repeated firing and the wear of the firearm barrel. As the firearm barrels heat up from repeated firing they will warp off axis due to residual stresses in the metal structure. This movement though ever so slight when measured at the muzzle becomes quite significant when measured at a target 200-300 yards away. In addition as the firearm barrels wear, their ability to maintain accuracy is severely diminished. Frequent replacement of conventional firearm barrels and components is necessary, particularly in bench rest shooting, varmint hunting, shooting teams, and the military. Firearm barrels and components treated with the controlled thermal profiling process of this invention have demonstrated that they have reduced residual stresses and increased wear resistance. This allows the firearm barrels and components to be fired with greater accuracy for longer periods of time." U.S. Pat. No. 5.865,913.
However, cryogenic process is laced with problems in aspects of how to best carry it out. For example, from the above-quoted patent on the firearms barrels--U.S. Pat. No. 5,865,913--it gave the warning that "sub-ambient treatments in the past utilized a liquid process which in some cases will cause thermal shock. This is detrimental as it will add stress to the structure." Id.
In U.S. Pat. No. 5,442,929--Gillin, a cryogenic treatment of electrical contacts is disclosed in which, the contacts-under-treatment are enclosed within a sheath, such as a layer of aluminum foil, "to cover the contacting surface and protect the contact from convection currents or other sources of thermal irregularities and to provide a uniform microclimate about the contact." U.S. Pat. No. 5,442,929.
U.S. Pat. No. 5,174,122--Levine, lists compound ways which cryogenic processing can go awry and diminish the wearability of a part rather than extend it. "Some of the problems encountered with the prior apparatus described above arise as follows:--(1 ) delivery of liquid nitrogen to the bottom of the chamber below the payload platform often splashes or splatters the liquid on the payload parts causing extreme thermal shock to the parts that are still relatively warm; (2) the coldest gas in the chamber is just above the liquid and the gas does not flow upward (rise) to the payload parts--the cold gas does not reach the parts until just about all of the gas in the chamber is cold and the coldest gas will always be below the payload parts; (3) pre-soaking the part partially submersed in the liquid nitrogen causes the part to chill unevenly, as the portion of the part that is submersed chills much faster than the portion that is not submersed; and (4) any submersion of the part in the liquid nitrogen results in boiling heat transfer from the part at an excessive rate that does not allow all portions of the part to cool evenly." U.S. Pat. No. 5,174,122.
The foregoing cautions about cryogenic problems are exponentially exacerbated when the article-under-treatment is ultra-small.
Here, the PCB drill bits range in diameter from between about 20/10,000-ths of an inch (0.0020 inches) and 1/4-th of an inch (0.250 inches).
Especially in the smaller sizes, any minute thermal irregularity which might not noticeably affect a drill bit measuring three (3) inches in diameter might just as likely render unfit for its intended use an ultra-small drill bit measuring 20/10,000-ths of an inch (0.0020 inches) in diameter. For perspective, that diameter is finer than human hair in most instances.
Accordingly, what is needed is a thermal treatment which incorporates a cryogenic process and which provides the advantages obtained but cryogenic process for large articles while avoiding the hazards that endanger the success of cryogenic process when applied to ultra-small articles.
These and other aspects and objects are provided according to the invention in a process for treating carbide tool bits used by the electronics industry for PCB fabrication combines a cryogenic cycle with two or more tempering cycles. The inventive process preferably comprises the following steps.
At the start, carbide tool bits as used by the electronics industry for PCB fabrication resting are found at rest in an ambient environment likely between about 65.degree. F. and 100.degree. F. The tool bits are subjected to a cryogenic cycle having a ramp down phase during which from an initial start time the tool bits are ramped down in a dry cryogenic environment to about -300.degree. F. over between about six (6) and eight (8) hours, followed by a cryogenic hold phase during which the tool bits are held at about -300.degree. F. over between about twenty-four (24) and thirty-six (36) hours, followed by a cryogenic ramp up phase during which the tool bits are ramped up to about -100.degree. F. over between about six (6) and eight (8) hours.
That is followed by a first tempering cycle having a ramp up phase during which the tool bits are ramped up in a dry tempering environment to about 350.degree. F. over about one-half (1/2) hour, followed by a hold phase during which the tool bits are held at about 350.degree. F. over about two (2) hours, followed by a ramp down phase during which the tool bits are ramped down to below about 120.degree. F. but not generally all the way to the ambient temperature over between about two (2) and three-and-half (31/2) hours. A second tempering cycle follows that and it has a time-temperature profile fairly comparable to the first tempering cycle. Optionally, a third tempering cycle can be included too.
The inventive process might have the cryogenic ramp down phase arranged such that it has a varying rate of descent that is more steep initially from ambient to about -100.degree. F. and then more gradual thereafter for temperatures below -100.degree. F. to about the cryogenic hold temperature of about -300.degree. F. The temperature descent from the start time at ambient temperature to the about -100.degree. F. level might be achieved over about the first one (1) hour after the start time. That way, the temperature descent from below about -100.degree. F. to about -300.degree. F. is achieved over between about five (5) and seven (7) hours.
The inventive process might have the cryogenic ramp up phase arranged such that it has a varying rate of ascent that corresponds to an exponential decay of the cryogenic hold temperature from the about -300.degree. F. to about -100.degree. F. over between the about six (6) and eight (8) hours therefor. The exponential decay of the cryogenic hold temperature from the about -300.degree. F. to about -100.degree. F. might transpire such that a temperature of about -200.degree. F. is not reached from the base hold temperature of -300.degree. F. until six (6) hours into the cryogenic ramp up phase, the remaining decay up to -100.degree. F. occurring over a next two (2) hours. Alternatively, the exponential decay of the cryogenic hold temperature from the about -300.degree. F. to about -100.degree. F. might be arranged to transpire such that a temperature of about -200.degree. F. is not reached from the base hold temperature of -300.degree. F. until five-and-half (51/2) hours into the cryogenic ramp up phase, the remaining decay up to -100.degree. F. occurring over a next half (1/2) hour.
Optionally, the cryogenic environment is provided by a Dewar chamber. The tempering environment might be provided by a convection oven. Accordingly, the transition between the cryogenic cycle and first tempering cycle would thus entail physical transfer of the tool bits from Dewar chamber to the convection oven.
A number of additional features and objects will be apparent in connection with the following discussion of preferred embodiments and examples.